The present invention relates to a process of continuously separating electrically charged, solid pulverulent materials by electrophoresis and electroosmosis. The invention is used on solid materials which, suspended in an aqueous medium and while subjected to an electric field, behave like electro-negative ions, the materials either being intrinsically electro-negative or made electro-negative by appropriate means.
Many of the processes developed in the mineral industry, particularly for processing china clays, require concentrating of suspensions which contain very fine or even colloidal particles. In general, the dehydration techniques, which resort to the classical operations of filtration, sedimentation, centrifuging, cyclone settling, and drying by heat, are either too inefficient as they do not make it possible to obtain a sufficiently dry solid, or have a prohibitive cost from the economic viewpoint. But for particles with a diameter not exceeding 10 microns, electrofiltration is an efficient way of concentrating, which involves for the same final result an energy equal to only one tenth of that required by a thermal drying operation.
The principle of electrofiltration is known per se. In this operation, a suspension containing the solid particles to be separated from the liquid carrier is subjected to an electric field generated between two electrodes. This implies, first of all, that the suspension to be treated can conduct electric current. When the solid particles are electro-negative, they migrate under the influence of the electric field toward the anode on which they tend to deposit. The liquid set free by the movement of the solid material moves in the opposite direction and therefore migrates with the electropositive ions present in the suspension toward the cathode. The movement of solid particles and of liquid in opposite directions causes separation of the two phases, with the solid phase being concentrated by deposition on the anode.
Of course, the efficiency of the operation depends upon a certain number of parameters only the most important of which will be cited:
dispersion of the particles within the surrounding liquid mass, the dispersion being a function of the specific weight of the particles and of their electrokinetic potential also termed zeta potential; PA1 mobility of the solid particles, which depends upon the zeta potential, the electric field applied to the particles, and the viscosity of the liquid in which the particles move; PA1 the medium's intrinsic electric resistivity which determines the amplitude of the current flowing in a given electric field. PA1 either protecting the anode by providing in its immediate vicinity a semi-permeable membrane defining an anode compartment of small volume, with the cake of solids deposited on the membrane to avoid in this way the contamination of the solids deposited by corrosion products from the anode (U.S. Pat. No. 4,048,038 and Addition Certificate 2,423,254 to French Patent 2,354,802); PA1 or utilising noncorrodible electrodes made from noble materials (e.g., tantalum) or from metals coated by electroplating with these noble materials (titanium, platinum) or from metal oxides not susceptible to corrosion. PA1 (continuously) introducing the suspension containing the electro-negative solid material into an electro-separation cell having an anode and a cathode between which an electric field is maintained, the cathode being provided with a filter medium permeable to only one liquid phase and defining a cathode compartment containing a catholyte, whereas the space between the filter medium and the anode is the treatment region of the suspension, from where the excess of the suspension is removed by appropriate means, PA1 moving the solid particles by an electric field, depositing the particles in the form of a cake on the anode surface, and unloading the cake outside the suspension-treatment region; PA1 displacing the liquid phase in the opposite direction, toward the cathode department, filtering through the filter medium under the influence of a reduced pressure, and removal from that compartment, PA1 a) into the cathode compartment there is discharged an amount Q.sub.3 of catholyte per unit time, with Q.sub.3 greatly exceeding Q.sub.T, PA1 b) the amount Q.sub.3 is split into two fractions Q.sub.1 and Q.sub.2, with Q.sub.1 being discarded, PA1 c) the fraction Q.sub.2 is continuously treated in a treatment device outside the electro-separation cell with the aid of a treatment agent, PA1 d) the treated fraction Q.sub.2 is continuously reinjected into the cathode compartment for the purpose of modifying the ratio of the species of ions present in the catholyte and of making the catholyte itself to a natural discardable product. PA1 the electric field strength between the electrodes is selected in the range 1-25 volts/cm and preferably in the range 5-15 volts/cm; PA1 the electric current density is adjusted to the range 1-20 milliamperes/cm.sup.2 and preferably to 5-16 milliamperes/cm.sup.2.
Electro-negative features and the importance of such features, which significantly influence the dispersion of the particles within the liquid phase, can originate from the use of mineral additives or water-soluble organic additives. These additives, which are introduced in very small amounts, adhere to the solid particles by absorption, make them more or less electro-negative, and therefore contribute to their dispersion within the liquid. Further, they may modify the intrinsic ionicity of the liquid and, hence, affect the resistivity of the medium.
Other parameters associated with the concept and the building of electric filtration cells determine the efficiency of operation of these cells. Among them one can cite the shape of the electrodes, their position, their spacing, and the type of the materials of which the electrodes are made.
The specific problems which are encountered by those skilled in the art in any electrofiltration operation are: obtaining, by deposition on the anode, sufficiently dry solids to be collected, and eliminating the liquid displaced by the concentration of the solid material deposit.
In order to increase the efficiency of the accumulation of the solids, the form of the anode has been modified in the course of time. For many years the use of rotary anodes in the form of a drum allowing the simultaneous continuous deposition and removal of solid cakes has been known, particularly from U.S. Pat. No. 1,133,967. The drum, the axis of which is horizontal, is half-way immersed in a tank containing the suspension to be concentrated. The solids are continuously collected by scraping with an appropriate device (knife, wire, scraper) from the anode surface while the latter emerges during its rotation from the suspension.
Similarly (see U.S. Pat. Nos. 3,972,799 and 4,107,026 and French Patent 2,552,096), the use of disk-shaped anodes facilitating a substantial increase in active surface at a given volume of the equipment has been known. These anodes are vertically mounted on a horizontal shaft and rotate semi-immersed in a tank supplied with the suspension to be treated. In constant intervals between each of the anodes there are mounted partitions integral with the tank, electrically insulated from it, and connected to the negative pole of a current generator.
Owing to the oxidation which takes place at the anode by release of oxygen and which can imply a degrading of the anode by corrosion, those skilled in the art have turned to:
Nevertheless, those skilled in the art always encounter the problem of continuously eliminating the water liberated by migration and electrolytic deposition of solids on the anode and, at the same time, of maintaining a constant concentration of the suspension present in the electro-separation cell.
The first cells did not comprise a semi-permable membrane which would have defined an anode compartment and a cathode compartment.
According to U.S. Pat. No. 1,132,967, the cathodes used are formed by metal elements in the form of rods or plates providing space for the passage of the liquid phase. Electrodes used by other inventors were a wire gauze wound upon a drum forming the cathode (U.S. Pat. No. 1,435,886). The anode formed by an endless ribbon mounted on rollers contacts the cake deposited on the drum and assumes its curvature. The production of a concentrated solid deposit by elimination of the water in the cake is furthered either by application of a contact pressure between the ribbon mounted on the rollers and the drum or by subjecting the drum to a negative pressure. For the purpose of increasing the efficiency of dehydration, one combines in this way the extraction of the liquid by electroosmosis and filtration either by applying an external pressure or by a pressure reduction.
Subsequently the search for higher efficiency led those skilled in the art to the use of semi-permeable membranes to form two clearly separated compartments, the anode compartment and the cathode compartment. The function of the semi-permeable membranes is to allow passage of the liquid and to remain impermeable to the passage of the solids. When the suspension to be treated has been introduced into the anode compartment and the electro-negative ions migrate toward the anode, the membrane acts as a filter medium with respect to the cathode. The liquid, the volume of which corresponds to the volume of the solids migrating toward the anode, is displaced by drag forces in the opposite direction, toward the cathode, passes through the filter medium, and enters into the cathode compartment. The passage of the liquid through the medium depends upon a certain number of factors associated, on the one hand, with the composition of the medium proper (type of material, porosity, permeability, . . . ) and, on the other hand, with the medium itself (viscosity of the liquid acting as the electrolyte of which water is the main component).
The passage of the liquid across the medium toward the cathode compartment can be facilitated, as indicated above, by a reduced partial pressure in this compartment (U.S. Pat. No. 4,003,849), and this is equivalent to increasing the acting pressure of the osmotic flow across the medium. Adjustment of the liquid extraction by adjusting the level of the catholyte has been claimed in U.S. Pat. No. 4,107,026. It is known that the water flow rate across the medium is related to the loss of charge of the flow. Since the desired goal is to have the solid particles migrate in the direction of the anode, there exists an electric field value, the so-called "critical value", beyond which the electric force exceeds the drag force upon the particle in equilibrium. The strength of the electric field applied therefore must be above this "critical value" to avoid that the finest particles, which are transported by the displacement of the liquid, gradually accumulate on the cathodic filter medium. By countering the flow through the medium, this accumulation makes the passage of the aqueous phase more difficult.
The entry of water into the cathode compartment by passage through the medium causes dilution of the electrolyte (termed catholyte) which is present in this compartment. The need for maintaining a certain concentration of the catholyte to ensure electric current flow from the cathode to the suspension to be treated has led the researchers to extracting the diluted electrolyte for the purpose of renewing it by bleeding it directly into the cathode compartment.
Adjustment of the discharge of liquid extracted from the cathode compartment has been claimed in French Patent 2,354,802 and in its Addition Certificate 2,423,254. The extraction of the aqueous phase from the cathode compartment is adjusted by continuously measuring the liquid level of the catholyte and influencing either independently or simultaneously the following two parameters: the reduced pressure above the liquid level of the cathode compartment, and the density of the electric current flowing through the electro-separation cell.
Indeed, the passage of liquid across the cathodic filter medium is facilitated by permanently maintaining a low pressure above the level of the catholyte. The partial vacuum above the cathode is adjusted with the aid of a vacuum pump controlled by the measurement of the low pressure. Furthermore, the development of a slight deposit of fine solid particles on the filter medium or the accumulation of those particles in its close vicinity can be enhanced or reduced by modifying the electric field strength and, hence, the current density. An increase in the current density causes a corresponding increase in the velocity of the solid particle migration toward the anode. A decrease in current density implies the development of a particle deposit on the filter medium, with the deposit opposing the passage of liquid. By simultaneously adjusting the two parameters (reduced pressure above the level of the catholyte and current density), those skilled in the art have achieved balanced operation of the cell for a fixed discharge rate of the filtrate out of the cathode compartment.
Nevertheless, Applicant's experience has shown that in the course of time, after a certain number of hours of operation, the regulation does not account for the fact that the two parameters mentioned above turn out to be insufficient for maintaining efficient operation of the electro-separation cell because one observes at the same time a progressively decreasing deposition of solids on the anode, a decrease in the discharge of the filtrate passing through the cathode medium, and an increase in the pH of the catholyte to high alkaline values.
In order to increase the efficiency of the separation of solids, those skilled in the art have resorted to the introduction of acid additives into the cathode compartment. From the above-cited U.S. patents (U.S. Pat. Nos. 3,980,547, 4,003,811, and 4,048,038) it has been known to use for this purpose mineral acids such as hydrochloric acid, sulfuric acid, or phosphoric acid. According to those patents, one tries to keep the pH of the catholyte between 2 and 7, with the concentration of the acid solution introduced changing from 0.1% to 10% by weight (U.S. Pat. No. 3,980,547) or from 0.1% to 1% by weight (U.S. Pat. Nos. 4,003,811 and 4,048,038).
To the extent to which the aqueous phase, which comes from the anode compartment, penetrates the membrane in the cathode compartment by electroosmosis, the ensuing dilution of the catholyte implies a change of the electric characteristics (specifically of the resistivity) of the latter. In order to maintain the electric current flow on an adequate amplitude, the properties of the catholyte and, hence, its composition must not vary beyond certain limits. In order to extract the aqueous phase as indicated above, and in order to maintain a steady current flow at the known amplitudes, those skilled in the art (U.S. Pat. Nos. 3,980,547, 4,003,811, and 4,048,038) have resorted to providing open continuous circulation of the catholyte across the cathode compartment. Fresh catholyte is continuously introduced directly into the cathode compartment and, the used catholyte is extracted from that compartment. As indicated above, an acid solution is admixed to the fresh catholyte to maintain the pH of the catholyte contained in the cathode compartment within the range 2-7.
In other prior-art embodiments, some electro-separation systems comprised a second semi-permeable membrane which is situated around the anode and defines a closed space termed anode compartment. The purpose of this membrane, which likewise forms a filter medium and on which the solid particles are deposited by electrophoresis, is to avoid contamination of the deposit by the corrosion products from the anode (U.S. Pat. No. 4,048,038).
The electrolyte contained in the anode compartment is termed anolyte and contains mineral salts some of which dissociate in the electrochemical reactions taking place near the anode. The resulting changes in the electrical properties require, as in the case of the catholytes, renewal of the anolytes for maintaining their composition close to constant values.
French Addition Certificate 2,423,254 has disclosed devices effecting this renewal. The system comprises a set of units arranged in the form of a closed loop on the anode compartment. The anolyte extracted first passes into a degassing chamber in which the gas generated by electrochemical decomposition of the electrolyte contacting the anode is separated from the liquid phase. Under the influence of the force of gravity, the liquid phase runs into a buffer vessel before being returned by a pump to the anode compartment. The used anolyte is directly extracted from this buffer vessel. Similarly, fresh anolyte contained in an intermediate storage tank is introduced at a point of the circuit upstream relative to the buffer vessel. Nevertheless, since the extraction of the used anolyte is made directly via an overflow limit set onto the buffer vessel through which the recycled anolyte flows toward the anode compartment, the system cannot work efficiently in the form described if the introduction of new anolyte into the circuit and the bleeding of the used anolyte are made discontinuously and independently of each other. As a consequence, the composition of the electrolyte in the closed circulation on the anode cannot be kept constant as described in the above-cited certificate and varies between limit values which have not been specified.
Further, since the anolyte is an NaCl solution, chlorine, which is released at the anode, is produced by electrolytic reaction. As indicated in French Addition Certificate 2,423,254, this chlorine must be eliminated or possibly reintroduced into the cathode compartment. But the handling of chlorine gas and its reintroduction into that compartment pose serious problems. When the chlorine is introduced into the atmosphere of the hydrogen liberated in the cathode compartment, an explosive mixture can develop. When the chlorine is directly injected into the catholyte, the injection implies the development of hypochlorous acid with a small dissociation constant which does not impede the gradual change of the catholyte to a highly alkaline pH. Further, the hypochlorites produced in the catholyte are aggressive compounds which destroy the cathode by corrosion and contaminate the catholyte the extracted fraction of which cannot be released into the natural environment.
Thus, whatever the teachings of the prior art in regard to treating the catholyte, namely the injection of chlorine or the introduction of mineral-acid solutions (HCl, H.sub.2 SO.sub.4, H.sub.3 PO.sub.4), the catholyte then contains foreign elements such as sulfur, chlorine, and phosphorous which initially were not present in the suspension to be treated. The fraction of the catholyte drawn from the cathode compartment is an effluent which, before it is discarded, requires an appropriate treatment so that the anti-pollution regulations are fulfilled.
It is scientifically known that the volume of the aqueous phase eliminated corresponds to the volume of the aqueous phase displaced by electroosmosis across the cake of the solid particles deposited on the anode. Therefore, continuous electro-separation, without sudden stops, requires at the same time perfect uniformity of the transition of the aqueous phase across the filter medium and excellent uniformity of its extraction.
Applicant has shown by his research that the hydroxyl (OH--) ions generated near the cathode by the electrolysis reaction are important. These ions, which migrate toward the anode, collide during their movement with the barrier which the cathodic filter medium forms and hinder the passage of the aqueous phase in the opposite direction through that medium.
The accumulation of hydroxyl ions in the cathode compartment therefore unfavourably affects the operation of the cell as the passage of the aqueous phase toward the cathode is impeded and, hence, the concentration of the solids deposited on the anode by electroosmosis is disturbed. This accumulation is converted into a change in the pH of the catholyte which develops to highly alkaline values. Therefore, after a certain number of hours of operation, this accumulation leads to a greatly impaired operation of the cell with the risk of blocking, regardless of the adjustment of the other parameters affecting electro-separation (electric field, current density, resistivity of the medium, low pressure on the cathode compartment, etc.).